In the communication industry, one type of common communication cable is formed from a pair of two wires twisted around one another, commonly referred to as a twisted pair. Typical high speed communication cables are comprised of a number of unshielded twisted pairs running through an outer jacket.
One problem that typically confronts the installation of such cables is that undesired capacitive and inductive coupling, also known as crosstalk, can occur between an unshielded twisted pair in a first cable with other items outside the cable, in particular with unshielded twisted pairs running in adjacent cables.
In order to reduce these unwanted conditions, prior art methods have introduced a number of changes into the cables, all with various degrees of satisfaction. For example, a first method used to reduce coupling with twisted pairs in adjacent cables is to increase the rate of twist between the conductors in the twisted pairs. However, by increasing the rate of twisting, the amounts of material used is greater per unit of distance, thus increasing the weight of the twisted pair, and the cable as well, and also leading to a greater amount of conductor losses in the signal due to the additional distance needed to be traversed.
A second method for addressing the condition of coupling with unshielded twisted pairs in adjacent cables is to simply increase the distance between them. In the prior art, this is done simply by increasing the thickness of the jacket. However, this presents a number of additional problems, all of which render the cable unfit.
For example, the additional material used for the jacket requires that more material be used. This additional material adds construction cost, adds weight to the final cable and also adds more fuel in the case of a fire, thus reducing or eliminating the ability of the cable to meet the required fire safety standards.
In addition to these basic physical constraints to simply adding more material to the jacket in order to prevent coupling with unshielded twisted pairs in adjacent cables, another drawback is that it will increase the amount of dielectric loss. This is particularly true with cables that include twisted pairs surrounded by a PVC jacket which is widely used for cable jacketing because of its low cost and fire resistant properties. Although PVC is commonly used for the above reasons, its poor dielectric properties also lead to increased loss in the unshielded twisted pairs. Thus, this condition is exacerbated when the jacket is made even thicker.
Another prior art solution was to place the jacket of the cable onto the twisted pairs in a loose fitting arrangement. Such a design, both increases the distance between the twisted pairs and outside interference sources and also reduces the amount of capacitive coupling, both of which are accomplished while maintaining the same amount of jacket material. However, this solution is inadequate because the loose fitting arrangement of the jacket allows the internal twisted pairs to vary their proximity to the jacket along the distance of the cable. This causes impedance variations along the length of cable as the internal twisted pairs move into and out of contact with the jacket.
Yet another solution, such as that proposed in U.S. Pat. No. 5,796,046, proposes an arrangement to add striations to the internal diameter of the jacket in order to generate a continuous and evenly spaced gap between the unshielded twisted pairs in the center and the bulk of the outer jacket. However, this design may suffer from a few drawbacks. First, by adding the striations, additional material is again included, adding weight, cost and reduced efficiency in meeting fire safety standards. Additionally, because the striations include a significant amount of material in and of themselves, having numerous contact points with the twisted pairs, there is still a significant amount of dielectric loss caused by the jacket.
In spite of past attempts to solve the problem of reducing coupling between unshielded twisted pairs in adjacent cables, there is still no low cost, light weight solution that also meets the necessary fire safety standards.